General-purpose air-cooled four-cycle engines have various requirements depending on what sort of working machine the engine is to serve. A compressor, an axial-flow pump, or an outboard engine directly connected to the propeller shaft requires high-speed revolution, so for these applications the crankshaft is used as the output shaft. Much farm equipment, on the other hand, requires low-speed output. Since the camshaft has a rotary speed half that of the crankshaft, it is used as the output shaft for this sort of application.
The crankshaft in an engine such as those described above, in which the crankshaft serves as the output shaft, is supported by ball bearings on both ends. These ball bearings must have a load rating which allows them to handle the radial load which the pistons receive from the pressure of combustion gases in the cylinder head as well as the additional radial and thrust loads imposed on the output shaft from the exterior. Generally, standard ball bearings with an intermediate load rating are used in these engines; and for the sake of interchangeability of parts and processes, the same size of bearing is generally used on both ends of the crankshaft.
A governor is essential in a multipurpose engine to adjust the r.p.m. Normally, a mechanical governor is enclosed in the crankcase and driven directly by the crankshaft. An example of such a governor is provided in Japanese Patent Publication (Kokai) Heisei 5-44522. We shall now discuss its configuration with reference to FIG. 3 below.
FIG. 3 shows the configuration of the components inside the crankcase. In the drawing, crankshaft 5 comprises crankshaft portions 5a.sub.1 and 5a.sub.2, crank arms 5b and crank pin 5c. The shaft portions 5a.sub.1 and 5a.sub.2 are supported at two points by ball bearings 40a and 40b, of identical external diameter, which are mounted on crankcase 17 and cover 18, respectively. Timing gear 3 is shaft-coupled to shaft portion 5a.sub.1 on the output end of the crankshaft 5. The rotary force of crankshaft 5 is transmitted to camshaft 7 by means of cam gear 8. Camshaft 7 is supported at two points by sliding bearings 70a and 70b, of identical external diameter, which are mounted on crankcase 17 and cover 18, respectively.
With the prior technology, gear 2, which drives the governor, is installed at the side of crankshaft portion 5a.sub.1 (output shaft), and the outer side (right side) of timing gear 3 as shown in FIG. 3. Small governor 10 is enclosed in space 70, which extends downward between the bottom of output shaft portion 5a.sub.1 in crankcase 17 and the bottom of crankcase 17 and cover 18. This design led to a demand that the size of the crankcase be reduced slightly so that the engine could be made smaller.
Some loads placed on the output shaft of this sort of crankshaft, for example an axial-flow pump or a propeller shaft directly coupled to an outboard engine, entail a large thrust load in the direction in which the output shaft pulls. For these loads, the bearing configuration used in the prior art, such that the crankshaft is supported at two points by ball bearings with the same exterior diameter, has insufficient bearing capacity, so that the bearings will not be durable over a long period of time.
If bearings of a larger diameter are used, the distance between the crankshaft and the camshaft increases, so the crankcase must be made larger. This goes counter to the goal of downsizing the crankcase.
It would be possible in this situation to use the same size of bearings but choose different materials, so that the bearings had a very precise fit and the bearing capacity was increased. However, such a special bearing would then be necessary to increase the precision of the fit with the crankshaft. This would entail selective fitting, which would pose a problem in terms of robotic assembly. Also, since standard specifications of bearings and this kind of special bearing have the same appearance in size, there would always be the chance of an assembly error by selecting wrong one.
Another problem with the existing design is that in addition to the cam gear and cam bearings, the governor and governor drive gear are also placed on the output end (right side in FIG. 3) of the crankshaft. No matter how much effort is put into reducing the size of the crankcase, space 70, which extends downward between the bottom of the crankshaft and the base of crankcase 17 and cover 18, is still needed. This makes the crankcase larger than is desirable.